Babies with Beckwith-Wiedemann syndrome (BWS) are overgrown and are at risk of birth defects, childhood cancers and other complications. Because of their large size, this can present as complications during the pregnancy including prematurity and difficult delivery putting both mother and baby at risk. The range of birth defects associated with BWS includes abdominal wall defects, an enlarged tongue, heart and genitourinary defects. These may require surgical correction shortly following birth or later on in childhood. The enlarged tongue may cause feeding and breathing difficulties initially and problems with speech later on. Babies with BWS and low blood sugars may also be at risk of seizures and brain damage. Children with BWS are also at increased risk of childhood cancers and require regular surveillance until the age of 8 by ultrasound scans and regular blood tests. The condition affects 1 in 13,700 people in the UK and it is estimated around 50 new cases are diagnosed in the UK each year. Children born following fertility procedures, including in vitro-fertilisation (IVF), have an increased risk of BWS. This project aims to identify new genes that affect growth and development and the development of childhood cancers by studying the DNA of patients with BWS extracted from a blood test. This research will help identify how each gene affects the various clinical features of BWS and should help the development of treatments and targeted cancer surveillance for these children. This could eventually allow new therapies to be developed to treat or prevent these features. Such information could also be relevant to other related disorders and children born following IVF. This work could also improve our understanding of more common diseases including cancer.